This invention pertains to a circuit arrangement for a reversible electric motor, in general and to a circuit arrangement in particular for a windshield wiper system.
Windshield wiper systems, in which the pendulum motion of the windshield wiper is directly derived from the rotary motion of the armature of an electric motor, whose polarity is reversed in the reversing positions and thus its direction of rotation is changed, have the advantage of being small and using less mechanical component parts. Moreover the wiping angle can be varied over wide ranges and thus be adapted to different windshield sizes. However the motional behavior of a windshield wiper driven by a reversible motor differs from the motional behavior of a windshield wiper of conventional systems as follows:
Because of the kinematic crank gear the wiper speed of conventional wiper systems is not constant. In contrast, the wiper speed of a wiper system comprising a reversible motor is constant, provided that the frictional conditions on the pane to be cleaned and the operating voltage are constant. Therefore, in a reversible motor driven system the wiper enters the reversing position at a relatively high speed, is then braked relatively abruptly and is subsequently started from the reversing position at a high rotational speed. Thus the motion of the wiper is not free from jerks. Some drivers dislike such motion. Moreover in conventional wiper systems the wiping angle is mechanically determined by the design of the crank gear. In contrast, in reversible motor wiper systems the wiping angle depends on various factors, because due to the kinetic energy the wiper is not immediately stopped when the motor is switched off, but moves on across a larger or smaller after-running angle. Thus the parking position limits of a reversible motor wiper system cannot be defined as narrowly as in conventional systems.
One circuit arrangement directed to solving this problem has been described U.S. Pat. No. 4,329,631. In that circuit arrangement the rotational speed of the motor is determined and, in dependence thereon, the motor is stopped at differing angular positions. The wiping angle can be held constant within narrow limits by a circuit arrangement of this kind, so that the parking position of the wiper can also be defined more exactly.